An oscillatory-type (oscillatory wave) actuator includes an oscillator that excites driving vibrations to a ring-shaped, prolate-ellipsoid-shaped, or rod-shaped elastic member when an electrical signal, such as AC voltage, is applied to a device, such as a piezoelectric element, that converts electrical energy to mechanical energy. An example of the oscillatory-type actuator suggested heretofore is an oscillatory wave motor in which the oscillator is moved relative to an elastic member in pressure-contact with the oscillator.
A general structure of a ring-shaped oscillatory wave motor is described below as one example.
A ring-shaped oscillatory wave motor includes a piezoelectric material that has an inner diameter and an outer diameter such that the entire perimeter equals to an integral multiple of a particular length λ. A plurality of electrodes are disposed on one surface of the piezoelectric material and a common electrode is disposed on the other surface of the piezoelectric material to form a piezoelectric element.
The plurality of electrodes include two drive phase electrodes, a detection phase electrode, and non-drive phase electrode. Electric fields of opposite directions are alternately applied at a λ/2 pitch to a piezoelectric material in each of the drive phase electrode portions to conduct a polarization treatment. Accordingly, the polarity of expansion and contraction of the piezoelectric material with respect to the electric field in the same direction is reversed every λ/2 pitch. The two drive phase electrodes are spaced from each other by a distance equal to an odd multiple of λ/4. Usually, a non-drive phase electrode is formed in this gap portion so that the piezoelectric material in this portion does not vibrate and short-circuited with the common electrode via short-circuiting wires or the like.
The detection phase electrode is an electrode for detecting the oscillation state of the piezoelectric material. A strain generated in the piezoelectric material in the detection phase electrode portion is converted into an electrical signal corresponding to the piezoelectric constant of the piezoelectric material and output to the detection phase electrode.
A wire for inputting and outputting power to this piezoelectric element is formed and a diaphragm composed of an elastic material is attached to form a stator. When AC voltage is applied to one of the drive phase electrodes of the stator, a standing wave having a wavelength λ occurs throughout the entire perimeter of the diaphragm. When AC voltage is applied only to the other drive phase, a standing wave occurs in a similar manner but the position of the standing wave is rotationally shifted in the circumferential direction by λ/4 with respect to the standing wave mentioned earlier.
A ring-shaped elastic member is brought into pressure-contact with a surface of the stator opposite to the diaphragm to form a ring-shaped oscillatory wave motor.
Another type of oscillatory wave motor is an oscillatory motor in which electrodes and a diaphragm are attached to inner and outer sides of a ring-shaped piezoelectric material. This type of motor can be driven by the rotation of a rotor in pressure-contact with the inner or outer side, the rotation being caused by expansion and contraction (vibration) of the piezoelectric material.
When AC voltages having the same frequency and a time phase difference of π/4 are applied to the respective drive phase electrodes of the oscillatory wave motor, standing waves are combined, and a travelling wave (wavelength λ) of bending vibrations travelling in the circumferential direction occurs in the diaphragm.
During this process, the points that lie on the rotor-side diaphragm undergo a type of elliptical motion and the rotor rotates due to the frictional force from the diaphragm in the circumferential direction. The direction of rotation can be reversed by switching the phase difference between the AC voltages applied to the respective drive phase electrodes between plus and minus.
A control circuit is connected to the oscillatory wave motor to form a driving control system that can control the speed of rotation. This control circuit includes a phase comparator that compares the phases and outputs a voltage value corresponding to the result of comparison.
When an oscillatory wave motor is driven, an electrical signal output from the detection phase electrode is input to a phase comparator along with an electrical signal applied to the drive phase electrode. The phase comparator outputs phase difference so that the degree of deviation from a resonant state can be detected. The data is used to determine the electrical signal applied to the drive phase electrode and to generate a desired travelling wave so that the rotation speed of the rotor can be controlled.
However, the value of voltage output from the detection phase electrode is usually larger than the input upper threshold voltage value of the phase comparator. Accordingly, the oscillatory wave motor control system disclosed in PTL 1 provides a mechanism (step-down circuit) between the detection phase electrode and the phase comparator to decrease the voltage to a logic level.